This invention relates to methods and apparatus for processing a received serial data signal, and more particularly to methods and apparatus for determining the phase of such a signal so that it can be reliably sampled to recover its data content.
Receiver circuitry may receive a serial data signal with no accompanying information about the timing of the individual data bits in that signal. This timing information is sometimes referred to as the phase of the signal. The receiver circuitry must recover the data from the received signal. To do this, the receiver circuitry needs to sample the received signal during each data bit interval (“unit interval”) in that signal to determine whether the signal currently represents a binary 1 or a binary 0. Information about the phase of the signal is needed so that the signal can be sampled at a time during each unit interval that will give reliable results. For example, it may be desired to sample the signal as close to the center of each unit interval as possible. Because the data signal is not received with any accompanying phase information, the receiver circuitry itself must derive the phase information it needs from the received data signal. The process of determining the phase of a received data signal may be referred to as phase alignment; and because the phase of the received signal may change over time, the phase alignment may need to be dynamic to keep the results current at all times.
A known dynamic phase alignment (“DPA”) technique includes producing several candidate clock signals, all of which have the same frequency (related to the frequency of the received data signal), and each of which has a unique phase. For example, there may be eight candidate clock signals, the phases of which are equally spaced over one clock signal cycle. Phase detector circuitry is used to compare the phase of transitions in the received data signal to the phase of transitions in one of the candidate clock signals. Assuming the phase detector does not detect a perfect phase match (as it almost never does), the phase detector circuitry keeps moving from one candidate clock signal to the next trying to find the candidate signal having the phase that will be best for use in timing the sampling of the data signal.
Typically the phase detector circuitry quickly finds what it regards as currently the best (or at least a very good) candidate clock signal for use in controlling data signal sampling. But continued operation of the phase detector circuitry also typically causes it to switch to a different candidate clock signal as it continues to search for the best such signal to use. The new choice may in fact be somewhat better or somewhat worse than the old choice. In either case, as long as there is still some phase mismatch, the search for the best signal continues, which may cause the phase detector to soon revert to its previous choice. In other words, even when the system is effectively at convergence, continued searching for a better candidate clock signal to use may cause the system to unproductively bounce back and forth between two choices, one of which may be better than the other, but either of which can be used with acceptably good results. Although systems having the foregoing characteristic can operate very well, the above-described bouncing or hunting can be undesirable. For example, it can increase noise in the system, and it can cause data signal interpretation errors that might not otherwise occur.
Examples of phase detector systems of the type described above are shown in such references as Aung et al. U.S. patent application Ser. No. 09/805,843, filed Mar. 13, 2001, Lee et al. U.S. patent application Ser. No. 10/059,014, filed Jan. 29, 2002, Lee et al. U.S. Pat. No. 6,650,140, Venkata et al. U.S. patent application Ser. No. 10/195,229, filed Jul. 11, 2002, Venkata et al. U.S. patent application Ser. No. 10/273,899, filed Oct. 16, 2002, Venkata et al. U.S. patent application Ser. No. 10/317,262, filed Dec. 10, 2002, Lui et al. U.S. patent application Ser. No. 10/454,626, filed Jun. 3, 2003, Venkata et al. U.S. patent application Ser. No. 10/349,541, filed Jan. 21, 2003, Venkata et al. U.S. patent application Ser. No. 10/637,982, filed Aug. 8, 2003, Asaduzzaman et al. U.S. patent application Ser. No. 10/668,900, filed Sep. 22, 2003, and Asaduzzaman et al. U.S. patent application Ser. No. 10/672,901, filed Sep. 26, 2003. These references also show examples of systems that can be modified in accordance with the principles of the present invention (e.g., to include the phase detector circuitry of this invention in place of the prior phase detector circuitry).